1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine seismic data processing.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface which then travels downward into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downward through a body of water overlying the subsurface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes and recorded.
The appropriate seismic sources for generating the seismic signal in land seismic surveys may include explosives or vibrators. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types, including a small explosive charge, an electric spark or arc, a marine vibrator, and, typically, a gun. The seismic source gun may be a water gun, a vapor gun, and, most typically, an air gun. Typically, a marine seismic source consists not of a single source element, but of a spatially-distributed array of source elements. This arrangement is particularly true for air guns, currently the most common form of marine seismic source.
The appropriate types of seismic sensors typically include particle velocity sensors, particularly in land surveys, and water pressure sensors (typically water pressure gradient sensors), particularly in marine surveys. Sometimes particle acceleration sensors are used in place of or in addition to particle velocity sensors. Particle velocity sensors and water pressure sensors are commonly known in the art as geophones and hydrophones, respectively. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, pressure sensors and particle velocity sensors may be deployed together in a marine survey, collocated in pairs or pairs of arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers have been proposed that contain water particle velocity sensors such as geophones or particle acceleration sensors such as accelerometers, in addition to hydrophones. The pressure sensors and particle motion sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable.
The resulting seismic data obtained in performing the survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. The processed seismic data is processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image the geologic subsurface. In order to identify locations in the Earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data. The process of constructing the reflector surfaces defining the subterranean earth layers of interest from the recorded seismic data provides an image of the earth in depth or time.
The image of the structure of the Earth's subsurface is produced in order to enable an interpreter to select locations with the greatest probability of having petroleum accumulations. To verify the presence of petroleum, a well must be drilled. Drilling wells to determine whether petroleum deposits are present or not, is an extremely expensive and time-consuming undertaking. For that reason, there is a continuing need to improve the processing and display of the seismic data, so as to produce an image of the structure of the Earth's subsurface that will improve the ability of an interpreter, whether the interpretation is made by a computer or a human, to assess the probability that an accumulation of petroleum exists at a particular location in the Earth's subsurface.
Dual sensor towed streamer reflection seismic data consist of pressure field and vertical particle velocity field records. A central element in the processing chain of seismic data is its separation into records containing only the upgoing and downgoing components of the pressure wavefields. This separation can be performed after transforming the data into the frequency-wavenumber (f−kx−ky) domain, taking both the difference between and the sum of, respectively the frequency-wavenumber spectrum of the pressure record and a scaled version of the frequency-wavenumber spectrum of the vertical particle velocity record, and dividing the resulting spectra by two. (Note that, by simply using the inverse of the previous scaling filter, one could alternatively obtain the upgoing and downgoing components of the vertical particle velocity wavefields by taking a sum of and a difference between, respectively, the frequency-wavenumber spectrum of the vertical particle velocity record and a scaled version of the frequency-wavenumber spectrum of the pressure record, and dividing the resulting spectra by two.) Inverse-transformation from the frequency-wavenumber domain back to the time-space domain yields the desired upgoing and downgoing wavefield components. In this process, only the vertical particle velocity record (or alternatively, only the pressure record) is changed by scaling. For non-evanescent energy, it is scaled in the frequency-wavenumber domain by a real filter which systematically increases with increasing wavenumber for a given frequency. However, spatial aliasing in the cross-streamer direction is all too common in marine seismic surveys. The, in the case of cross-streamer aliasing, energy is wrapped to a lower cross-streamer wavenumber ky. Subsequently, if these wrap-around effects in the wavenumber are not taken into account, then the scaling filter is computed from the wrong wavenumber, one that is too low. Thus, the aliased energy in the vertical particle velocity record (or alternatively, the pressure record) is scaled by filter coefficients that are consistently too low (or too high, respectively).
The superposition of upgoing and downgoing wavefield components in the original records causes a specific pattern of receiver ghost notches in the corresponding frequency-wavenumber spectra. Whenever the recorded energy is cancelled at a specific frequency-wavenumber combination in, for example, the spectrum of the pressure field, the corresponding recorded energy is maximal in the spectrum of the vertical particle velocity. However, this correspondence causes an incorrect separation of the upgoing and downgoing wavefield components at the notches in the frequency wavenumber spectrum of the pressure record (or, alternatively, the vertical particle velocity record). At these locations, the aliased energy is incorrectly scaled for the vertical particle velocity record (or alternatively, the pressure record), so that the resulting separated wavefield components are incorrectly computed.
Thus, a need exists for a method for separation of upgoing and downgoing wavefield components in 3D dual sensor towed streamer seismic data, which properly handles aliased energy in the cross-streamer direction.